40K the first year will be tough unless your client base grows quickly from the get go. I'm not familiar with the type of grasses you will be mowing and the interval you will be mowing at. Most of my clients are every two weeks with a few that are mowed every ten days and even fewer that are weekly. Basically I figure that each client averages out to about 1K per year counting mulch, pine straw, etc.
Starting out it will be hard to justify hired help which would help out with the physical part. If you could make it on 20K the first year and work to build a satisfied client base, next year you should exceed 40K. Really what you make is only limited by your ability to gain customers and keep them happy. Hopefully you can tough it out, grow the business and eventually let the employees handle the physical part, while you concentrate on quality control, sales, cost control and other business issues.